This invention generally relates to semiconductor integrated circuits and more particularly to CMOS integrated circuits having circuit means for reducing electromagnetic emissions.
In modern electronic systems, which often include many integrated circuits, the rapid switching of many thousands of logic elements causes electromagnetic emissions. Such emissions, when transmitted along power buss conductors, can interfere with the operation of the electronic system. Electromagnetic emissions may also interfere with the operation of nearby electrical equipment due to electromagnetic fields radiated into space.
Electromagnetic fields are a combination of alternating electric and magnetic fields. The electric lines of force are perpendicular to the magnetic lines of force at every point in space. The field propagates in a direction perpendicular to both electric and magnetic lines of force.
The effects of electromagnetic emissions may be as simple as static on the radio or as catastrophic as complete failure of the electronic system. If the electronic system produces excessive emissions, the system may not meet government regulations regarding emissions standards set by, for example, the Federal Communications Commission (FCC).
Prior art electronic systems have attempted to reduce electromagnetic emissions with careful attention to the layout of printed circuit boards. Sometimes additional components, such as RLC filters or chokes, must be added to the power buss or other "noisy" conductors. If the electronic system includes software, such software may be rewritten to spread noise generating circuit activity over a longer time period. In extreme instances, the crystal frequency of the system clock may have to be reduced to increase the amount of time between bursts of electromagnetic emissions.
Additional prior art methods attempt to reduce the effect of electromagnetic emissions by shielding electromagnetic sensitive components with a Faraday cage or locating sensitive components away from the source of the electromagnetic emission.
Using all the above methods to reduce electromagnetic emission problems may be required to overcome susceptibility to such emissions on a single electronic system. However, due to the random nature of such problems, it is typically very difficult, time consuming and expensive to reduce system susceptibility.
Furthermore, as such electronic systems operate at higher frequencies, the generation of electromagnetic emissions may increase rendering the above methods insufficient to reduce system susceptibility and pass FCC regulations. Accordingly, there is a great need to reduce the generation of electromagnetic emissions at the source.
One major source of electromagnetic emissions in a typical modern electronic system is the integrated circuit. Such integrated circuits have large numbers of outputs, each of which switches a relatively large amount of current, and tens of thousands of internal, or nucleus, logic elements. With a high frequency clock signal, such integrated circuits generate high amplitude transient waveforms superimposed on the power supply busses or conductors. These waveforms, which include high amplitude, high frequency fundamentals and harmonics, can be radiated or conducted to other components in the electronic system.
The integrated circuit package can further exacerbate the problems associated with electromagnetic emissions. For example, packages, such as a pin grid array package, can increase conducted and/or radiated electromagnetic emissions because longer bonding wires are required to interconnect the bonding pad on the package to the bonding pad on the integrated circuit. Each of these bonding wires acts as a loop or whip antenna from which electromagnetic emissions are radiated. Bonding wires connecting output buffers of the integrated circuit to bonding pads on the package are major contributors to radiated emissions because of the large amount of current typically switched by an output buffer.
Several methods are typically implemented to reduce package induced electromagnetic emissions. One method uses a discrete capacitor, such as flip-chip capacitor, connected between each noisy conductor and ground. These capacitors, which are often placed within the package cavity, act as a low pass filter but since the resistive component is necessarily low, the filter is relatively inefficient. Another method requires that ferrite beads be placed around the outside of noisy package pins.
Both techniques often reduce electromagnetic emissions but may not provide sufficient reduction for high speed integrated circuits having a significant number of outputs. Further, positioning capacitors within package cavity is a non-trivial task significantly increasing the cost of such packages. Ferrite beads are similarly undesirable because a bead must be used for each pin generating electromagnetic emissions. Because of the cost associated with ferrite beads, their use is usually restricted to the package pins generating the most electromagnetic emissions.